In the cat spinal cord we have demonstrated a great degree of specificity in the connections between single Ia fibers and homonymous and heteronymous motoneurons. We have also demonstrated that axotomized motoneurons lose connections from Ia fibers. We wish to study axotomized motoneurons further to understand whether specific Ia connections are lost and to what extent this reaction represents a loss of a specific influence from the muscle. In addition, we will investigate whether reorganization of Ia-motoneuron connections occurs just caudal to a spinal cord hemisection by comparing data obtained in hemisected animals to our extensive observations in normal animals. Our aims here are twofold; first to examine neuronal plasticity at a single synapse level, and second to attempt to account for the changes which occur in this clinically interesting situation. We have previously shown in Xenopus laevis that supernumerary hindlimbs grafted to tadpoles and innervated by thoracic sensory fibers exhibit area specific reflex behavior. Touching the thoracically innervated grafted limb elicits appropriate behavior from the normal lumbar innervated hindlimbs. We would like to establish directly whether thoracic neurons establish atypical connections in the lumbar spinal cord. We will use physiological and anatomical techniques to study these questions. These will involve initial studies on connections in normal animals. In addition we would like to investigate more closely the identity of the neurons which innervate the supernumerary limb. We expect to use tritiated thymidine autoradiography to characterize the origin of neurons in Xenopus dorsal root ganglia in general and those neurons innervating the supernumerary limb in particular.